Absorption of mercaptans

ABSTRACT

Absorption solvents for removing mercaptans from gas streams are disclosed. The absorption solvents comprise an alkylether of a polyalkylene glycol, e.g., methoxytriglycol, and a secondary monoalkanolamine, e.g., N-methylethanolamine, as well as optionally other amines, e.g., methyldiethanolamine and diethanolamine. The absorption solvents do not require the presence of iodine for removal of mercaptans. Absorption processes utilizing the solvents are also disclosed.

This application is a division of application Ser. No. 08/501,702, filedJul. 12, 1995, now issued as U.S. Pat. No. 5,589,149, which is acontinuation of application Ser. No. 149,077, filed Nov. 9, 1993, nowabandoned.

FIELD OF THE INVENTION

The present invention relates to the removal of mercaptans from gasstreams. More specifically, the present invention relates to processesand solvents for removing mercaptans from gas streams by absorption.

BACKGROUND OF THE INVENTION

It is often desirable to remove acid gases, such as, for example, CO₂,H₂ S, SO₂, CS₂, HCN, COS, and sulfur derivatives of C₁ to C₈hydrocarbons, from gas streams. Gas streams from which these acid gasesmust be removed can be from many sources. One common source of such gasstreams is from natural gas wells. The gas removed from natural gaswells is often rich in methane and other combustible gases, but containsconcentrations of acid gases such as H₂ S, CO₂ and the other acid gasesdescribed above. High concentrations of H₂ S inhibit pipe line shipmentof the natural gas because of environmental considerations andgovernment regulation. High concentrations of CO₂ in natural gas reducethe heating value of the gas because CO₂ is not combustible. Mercpatans,i.e., sulfur derivatives of C₁ to C₈ hydrocarbons, have an offensiveodor and are corrosive.

The removal of mercaptans can be particularly difficult. One processproposed for the removal of mercaptans from a gas stream is described inU.S. Pat. No. 3,716,620, issued Feb. 13, 1973. The process includes thestep of contacting a gas containing hydrogen sulfide or a mercaptan witha solution of iodine in an organic solvent, e.g., an ether of apolyalkylene glycol, and an amine. The presence of iodine in processessuch as described in the above-referenced patent is generallyundesirable because the iodine must be regenerated in an oxidationprocess which increases the complexity and adds cost to the overall acidgas removal process. Accordingly, processes and absorption solvents aredesired for the removal of mercaptans from gas streams by absorptionwhich do not require the presence of iodine or suffer the disadvantagesdescribed above.

SUMMARY OF THE INVENTION

In accordance with the present invention, improved processes andabsorption solvents are provided for the removal of mercaptans from gasstreams. The absorption solvents utilized in the processes are highlyeffective for the absorption of mercaptans at low solvent circulationrates. The absorption solvent comprises:

(i) from about 10 to 98 weight percent based on the weight of theabsorption solvent on an anhydrous basis, i.e., excluding water, of analkyl ether of a polyethylene glycol of the formula:

    R.sub.1 --O--(CH.sub.2 CH.sub.2 O).sub.x --R.sub.2

wherein:

R₁ is an alkyl group having 1 to about 4 carbon atoms;

R₂ is hydrogen or an alkyl group having 1 to about 4 carbon atoms; and

x is 1 to about 10; and

(ii) from 1 to about 20 weight percent based on the weight of theabsorption solvent on an anhydrous basis of a secondary monoalkanolamine of the formula: ##STR1## wherein;

R₃ is an alkyl group having 1 to about 6 carbon atoms; and

R₄ is an alkyl group having 1 to about 4 carbon atoms.

In addition the solvent may contain from about 0.1 to 80 weight percentwater based on the total weight of the absorption solvent, andoptionally other amines such as, for example, dialkanol amines. Theabsorption solvent is further characterized by having less than 0.005moles of iodine per liter of absorption solvent.

By virtue of the present invention, it is now possible to removemercaptans from gas streams using a solvent containing an alkyl ether ofa polyethylene glycol and a secondary monoalkanolamine without the useof iodine. As a result, the processes of the present invention can offerthe following advantages, for example, over a process which requires thepresence of iodine: no oxidative regeneration step is required to reusethe iodine, thereby reducing the complexity and cost of the process; andsolids formation caused by the reaction of hydrogen sulfide with iodineto form sulfur is eliminated thereby reducing fouling of equipmentsurfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a process flow diagram of an absorption process inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The alkyl ether of the polyalkylene glycol of the present invention hasthe formula

    R.sub.1 --O--(CH.sub.2 CH.sub.2 O).sub.x --R.sub.2

wherein:

R₁ is an alkyl group having 1 to about 4 carbon atoms;

R₂ is hydrogen or an alkyl group having 1 to about 4 carbon atoms; and

x is 1 to about 10.

R₁ is preferably CH₃, C₂ H₅ or C₃ H₇, and more preferably CH₃.

X is preferably 2 to 8, more preferably 2 to 4 and most preferably 3. Inone aspect of the invention, R₂ is hydrogen. In another aspect of theinvention R₂ is preferably CH₃, C₂ H₅ or C₃ H₇.

When R₂ is hydrogen, typical compounds within the formula describedabove include, for example, methoxytriglycol, methoxytetraglycol,butoxytriglycol, ethoxytriglycol, methoxydiglycol, and butoxydiglycol.

When R₂ is an alkyl group, typical compounds within the formuladescribed above, include, for example, diethylene glycol diisopropylether, triethylene glycol diisopropyl ether, tetraethylene glycoldiisopropyl ether, polyethylene glycol dimethyl ether, polyethyleneglycol methyl isopropyl ether, polyethylene glycol methyl tertbutylether and propylene carbonate.

Methoxytriglycol and polyethylene glycol dimethyl ether are preferredalkyl ethers of polyethylene glycols for use in accordance with thepresent invention.

Methods for preparing alkyl ethers of polyethylene glycol suitable foruse in accordance with the present invention are known to those skilledin art. Alternatively, such compounds are available, for example, fromUnion Carbide Corporation, Danbury, Con.

Typically, the alkyl ether of the polyethylene glycol will comprise fromabout 10 to 98 weight percent of the absorption solvent based on theweight of the absorption solvent on an anhydrous basis. Preferably, thealkyl ether of the polyethylene glycol absorption solvent will comprisefrom about 20 to 95 weight percent, and more preferably from about 30 to90 weight percent of the absorption solvent based on the weight of theabsorption solvent on an anhydrous basis. Typically when R₂ is hydrogen,the alkyl ether of the polyethylene glycol will range from about 30 to60, weight percent of the absorption based on the weight of theabsorption solvent on an anhydrous basis. Typically when R₂ is an alkylgroup, the alkyl ether of the polyethylene glycol will range from about50 to 90 weight percent of the absorption solvent based on the weight ofthe absorption solvent on an anhydrous basis.

The secondary monoalkanol amine of the present invention has the formula##STR2## wherein;

R₃ is an alkyl group having 1 to about 6 carbon atoms; and

R₄ is an alkyl group having 1 to about 4 carbon atoms.

R₃ is preferably CH₂,C₂ H₄ or C₃ H₆, and more preferably, C₂ H₄.

R₄ is preferably CH₃ C₂ H₅ or C₃ H₇, and more preferably, CH₃.

Typical amines included within the above formula include, for example,N-methylethanolamine and N-ethylethanolamine. A preferred secondarymonoalkanolamine for use in accordance with the present invention isN-methylethanolamine.

Typically, the secondary monoalkanolamine will be present in theabsorption solvent in an amount from about 1 to 20 weight percent basedon the weight of the absorption solvent on an anhydrous basis.Preferably, the absorption solvent will contain from about 1 to 15weight percent, and more preferably from about 2 to 10 weight percent,of a secondary monoalkanolamine based on the weight of the absorptionsolvent on an anhydrous basis.

Other amines can also be included in the absorption solvents of thepresent invention. Preferably, such other amines are secondary ortertiary dialkanolamines such as, for example, methyldiethanolamine,ethyldiethanolamine, methylethanolpropanolamine,ethylethanolpropanolamine, and methyldipropanolamine. Trialkanolamines,such as, for example, triethanolamine may also be used in the absorptionsolvents of the present invention. When such dialkanol and trialkanolamines are employed, their concentration is preferably from about 10 to60 weight percent, and more preferably from about 20 to 50 weightpercent, based on the weight of the absorption solvent on an anhydrousbasis.

In a preferred aspect of the invention, the absorption solventspreferably comprise from about 10 to 50 weight percent, and morepreferably from about 10 to 40 weight percent, of diethanolamine basedon the weight of absorption solvent on an anhydrous basis. It is alsopreferred that the absorption solvents comprise from about 5 to 20weight percent, and more preferably from about 10 to 20 weight percent,of methyldiethanolamine based on the weight of absorption solvent on ananhydrous basis.

Methods for preparing the above-described amines are known to thoseskilled in the art. Alternatively, such amines are commerciallyavailable such as, for example, from Union Carbide Corporation, Danbury,Con.

The absorption solvents of the present invention are typicallyaqueous-based and often comprise from about 0.1 to 80 weight percentwater based on the total weight of the absorption solvent, i.e.,including water. Preferably, the absorption solvents of the presentinvention comprise from about 1 to 50 weight percent water based on thetotal weight of the absorption solvent. Often, the water concentrationranges from about 1 to 30 weight percent, is occasionally is less than20 percent and is sometimes from about 1 to 15 weight percent based onthe total weight of the absorption solvent. Appropriate waterconcentrations for the particular process conditions, feed gascomponents, and the like, can be determined by those skilled in the art.

Often, the absorption solvents of the present invention willadditionally contain additives such as, for example, corrosioninhibitors, defoamers, and the like. Typically, the concentration ofsuch additives will range from about 0.01 to 5 weight percent based onthe weight of the absorption solvent on an anhydrous basis. Furtherdetails concerning such additives are known to those skilled in the art.

Quite surprisingly, it has been found in accordance with the presentinvention that the above-described absorption solvents have capacity andselectivity for mercaptans without requiring the presence of iodine.Preferably, the amount of iodine in the absorption solvents of thepresent invention is less than about 0.005 moles of iodine per liter ofabsorption solvent and more preferably, less than 0.001 moles of iodineper liter of absorption solvent. Most preferably, there is an essentialabsence of iodine, i.e., less than about 100 parts per million on avolume basis.

Essentially any feed gas containing mercaptans can be used in theprocesses of the present invention. Typically, however, the feed gasstreams will contain mercaptans having from 1 to about 8, preferably 1to about 4, carbon atoms, CO₂, H₂ S, COS, hydrocarbons having from about1 to 4 carbon atoms, e.g. methane to butane, and water. It is notuncommon for the feed gas streams to also contain SO₂, SO₃, CS₂, HCN,oxygen and nitrogen. Typically, the mercaptans will be present in anamount from about 10 to 10,000 ppm_(v), often from about 10 to 2,000ppm_(v). H₂ S is typically present in a concentration of from about 0 to90 mole percent, often from about 4 ppm_(v) to about 50 mole percent.CO₂ is typically present in an amount of from about 0 to 50 mole percentoften from about 10 to 30 mole percent. COS, when present, willtypically comprise from about 2 to 10,000 ppm_(v). The hydrocarbonshaving from 1 to about 4 carbon atoms per molecule are typically presentin an amount of from about 10 to 98 mole percent. The sources of suchfeed streams are not critical to the present invention but include, forexample, natural gas wells, refinery coker off-gas, refinery fluidcatalytic cracker off-gas and other refinery gas streams.

The invention is hereafter described with reference to FIG. 1 whichillustrates a process flow diagram in accordance with the presentinvention. The process flow diagram is provided for illustrativepurposes and is not intended to limit the scope of the claims whichfollow. Those skilled in the art will recognize that the process flowdiagram does not illustrate various common pieces of process equipmentsuch as, for example, heat exchangers, pumps, compressors, heaters,process control systems and the like.

A feed gas stream containing 93 mole percent methane, 3 mole percentethane, 1 mole percent propane, 1 mole percent CO₂, mole percent H₂ S,13 milligrams per cubic meter (13 mg/m³) of methyl mercaptan, 108 mg/m³of ethyl mercaptan, 83 mg/m³ propyl mercaptan and 45 mg/m³ butylmercaptan were introduced to absorption zone 100 via line 10. Absorptionzone 100 comprises a gas-liquid contacting tower containing suitabletrays or packing material to conduct an absorption process. The detailsconcerning the apparatus used in the absorption zone are known to thoseskilled in the art. The absorption zone is typically operated at atemperature of from about 25° to 90° C. and a pressure of from about 100to 7000 kilopascals.

In absorption zone 100, the feed gas stream is contacted with a leansolvent stream introduced via line 11. The lean solvent stream comprisesan absorption solvent containing 50 weight percent methoxytriglycol, 5weight percent N-methylethanolamine, 30 weight percent diethanolamineand 15 weight percent methyldiethanolamine based on the weight of theabsorption solvent on anhydrous basis. The absorption solvent alsocontained about 38 weight percent water based on the total weight of theabsorption solvent. Typical solvent to feed ratios in the absorptionzone range from about 0.5 to about 3.5 liters of solvent per cubic meterof feed gas (l/m³) at standard conditions, i.e., one atmosphere and 0°C. Quite surprisingly, the absorption solvents have high capacity formercaptans at low solvent to feed ratios. For example, the capacity formethyl mercaptan removed is preferably from about 90 to 100% of themethyl mercaptan in the feed gas stream at a solvent to feed ratio ofless than 1.6 l/m³.

A product gas stream which is at least partially depleted in themercaptans relative to the feed gas stream is discharged from absorptionzone 100 via line 12. Preferably, from about 50 to 100 percent of themethyl mercaptan, from about 20 to 80 percent of the ethyl mercaptan,from about 20 to 85 percent of the propanol mercaptan and from 20 to 90percent of butyl mercaptan are removed from the feed gas stream inabsorption zone 100.

Quite advantageously, the absorption solvent of the present inventionalso has absorption capacity for H₂ S in addition to mercaptans.Moreover, the enhanced mercaptan removal capability of the absorptionsolvents of the present invention does not have an adverse impact on theH₂ S removal capability. Accordingly, preferably the product gas streamremoved via line 12 is also at least partially i.e., at least 50%, andmore preferably largely, i.e., at least 80%, depleted in H₂ S relativeto the feed gas stream. Also, the absorption solvents of the presentinvention have high capacity for CO₂ as well. Thus, preferably theproduct gas stream is at least partially depleted in CO₂ relative to thefeed gas stream.

A rich solvent stream comprising the absorption solvent and at least aportion of the mercaptans is withdrawn from absorption zone 100 via line13. The rich solvent stream also contains absorbed hydrogen sulfide andcarbon dioxide. The rich solvent stream is introduced to a regenerationzone 200 wherein the mercaptans, H₂ S and CO₂ are desorbed from theabsorption solvent. Regeneration zone 200 comprises a distillation/steamstripping tower containing suitable trays or packing material to desorbthe absorbed add gases. Details concerning the apparatus in regenerationzone 200 are known to those skilled in the art. Regeneration zone 200 istypically operated at a temperature from about 100° to 130° C. and apressure from about 100 to 400 kilopascals.

A tail gas stream comprising mercaptans, CO₂ and H₂ S is discharged fromregeneration zone 200 via line 14 and passed to condenser 300. A portionof the condensed tail gas stream is returned to regeneration zone 200via line 15 and the remainder is removed from the process via line 16.

A regenerated solvent stream comprising the absorption solvent, which isdepleted in mercaptans, H₂ S and CO₂ relative to the rich solventstream, is withdrawn from regeneration zone 200 via line 17. A portionof the regenerated solvent stream is passed to reboiler 400 andintroduced to regeneration zone 200 via line 18. The remainder of theregenerated solvent stream is recycled to absoption zone 100 via line 11as described previously.

EXAMPLE

The following example is provided for illustrative purposes and is notintended to limit the scope of the claims which follow.

A process such as described above was operated with the three absorptionsolvents having the compositions set forth below in Table 1. The percentremoval of methyl mercaptan for various solvent to feed ratios ("L/G")is also set forth in Table 1. The absorption zone was operated at atemperature of 50° C. and 40° C. for the absorption solvents of thepresent invention ("Solvent A" and "Solvent B") and 25° C. for thecomparative absorption solvent ("Solvent C"). The lower absorptiontemperature used for the comparative absorption solvent would beexpected to enhance the mercaptan removal capability of the comparativeabsorption solvent as compared to the higher absorption temperature usedfor the absorption solvents of the present invention.

                  TABLE 1                                                         ______________________________________                                                 SOLVENT A                                                                              SOLVENT B   SOLVENT C                                       ______________________________________                                        COMPONENT,                                                                    WT %                                                                          Polyethylene glycol                                                                      0          82.5        0                                           dimethylether                                                                 Methoxytriglycol                                                                         31         0           35                                          N-Methylethanol-                                                                         3          8.6         0                                           amine                                                                         Methyldiethanol-                                                                         9          0           35                                          amine                                                                         Diethanolamine                                                                           19         0           0                                           Water      38         8.9         30                                          METHYL                                                                        MERCAPTAN                                                                     REMOVAL, % OF                                                                 FEED L/G                                                                      0.8        100        --          --                                          10         --         90          --                                          16         --         86-92       74                                          1.7        94         --          --                                          2.5        --         --          87                                          2.9        --         --          92                                          3.4        100        79-90       --                                          ______________________________________                                    

The data in Table 1 show that by adding a secondary monoalkanolamine,e.g., N-methylethanolamine, to Solvent A, there was a significantimprovement in the removal of methyl mercaptan as compared to Solvent Cwhich did not contain a secondary monoalkanolamine. Similiarly,significant improvements (not shown) were also observed for Solvent Awith respect to the removal of ethyl mercaptan, propyl mercaptan andbutyl mercaptan, as well as COS as compared to Solvent C. Solvent Bprovided a higher degree of removal of methyl mercaptan, particularly atlow solvent to feed ratios, e.g., I/G of 1.6 and lower, than Solvent C.

Those skilled in the art will recognize that although the invention hasbeen described with respect to specific aspects, other aspects notspecifically described herein are intended to be included within thescope of the claims which follow. For example, amines, glycol ethers andadditives other than those specifically described herein can be includedin the absorption solvents of the present invention. Similarly, processvariations such as for example, utilizing multiple absorption zones withseparate lean solvent introductions or flashing zones to assist inregenerating the absorption solvent can be employed in the processes ofthe present invention.

We claim:
 1. An absorption solvent for use in an acid gas scrubbingprocess, consisting essentially of:(i) from about 30 to 60 weightpercent of the absorption solvent on an anhydrous basis ofmethoxytriglycol; (ii) from about 1 to 15 weight percent of theabsorption solvent on an anhydrous basis of N-methylethanolamine; (iii)from about 10 to 40 weight percent of the absorption solvent on ananhydrous basis of diethanolamine; (iv) from about 5 to 20 weightpercent of the absorption solvent on an anhydrous basis ofmethyldiethanolamine; and (v) from about 0.1 to 80 weight percent waterbased on the total weight of the absorption solvent.